James W HopperDepartment of Psychiatry, Boston University School of Medicine, Boston, MA, United StatesJ Psychosom Res 60:83-90. 2006

..Additionally, abnormally low tonic parasympathetic activity on the heart has been implicated in cardiovascular disease and hypertension. This study addressed a potential parasympathetic contribution to elevated basal HR in PTSD...

Disorders of extreme stress: The empirical foundation of a complex adaptation to trauma